Effects of microstructure characteristics on the tensile properties and fracture toughness of TA15 alloy fabricated by hot isostatic pressing

Powder hot isostatic pressing(HIP) is an effective method to achieve near-net-shape manufacturing of high-quality complex thinwalled titanium alloy parts, and it has received extensive attention in recent years. However, there are few reports about the microstructure characteristics on the strengthening and toughening mechanisms of powder hot isostatic pressed(HIPed) titanium alloys. Therefore, TA15powder was prepared into alloy by HIP approach, which was used to explore the microstructure characteristics at different HIP temperatures and the corresponding tensile properties and fracture toughness. Results show that the fabricated alloy has a “basket-like structure” when the HIP temperature is below 950℃, consisting of lath clusters and surrounding small equiaxed grains belts. When the HIP temperature is higher than 950℃, the microstructure gradually transforms into the Widmanstatten structure, accompanied by a significant increase in grain size. The tensile strength and elongation are reduced from 948 MPa and 17.3% for the 910℃ specimen to 861 MPa and 10% for the 970℃ specimen.The corresponding tensile fracture mode changes from transcrystalline plastic fracture to mixed fracture including intercrystalline cleavage.The fracture toughness of the specimens increases from 82.64 MPa·m^(1/2)for the 910℃ specimen to 140.18 MPa·m^(1/2)for the 970℃ specimen.Specimens below 950℃ tend to form holes due to the prior particle boundaries(PPBs), which is not conducive to toughening. Specimens above 950℃ have high fracture toughness due to the crack deflection, crack branching, and shear plastic deformation of the Widmanstatten structure. This study provides a valid reference for the development of powder HIPed titanium alloy.

Mechanical properties and microstructure of an α+β titanium alloy with high strength and fracture toughness

The Ti-Al-Sn-Zr-Cr-Mo-V-Si （Ti-62A） alloy, an alpha-beta alloy with high strength and fracture toughness, is currently used as an advanced structural material in aerospace and non-aerospace applications. Thermo-mechanical processes can be used to optimize the relationship between its strength and fracture toughness. A Ti-62A alloy bar can be machined through a transus β-forged plus α＋β solution treated and aged specimen with a lamellar alpha microstructure. The effects of heat treatment on the mechanical properties were discussed. Heat treatment provided a practical balance of strength, fracture toughness, and fatigue crack growth resistance. A comparison of the Ti-62A alloy with the Ti-62222S alloy under the same thermo-mechanical processing conditions showed that their properties are at the same level.

Effect of titanium addition on fracture toughness behavior of ZL108 alloy

Two different titanium alloying methods were applied to ZL108 alloy for preparing specimens containing titanium. The specimens were tested on the MTS 810 material test system for studying their behavior of the plane strain fracture toughness KIC.The experimental data were analyzed by the statistical significance tests. The results show that the fracture toughness of the ZL108 alloy containing titanium is superior to that of common ZL108 alloy containing no titanium, but there is no significant difference for different titanium alloying methods. Therefore titanium addition is an effective method for improving the fracture toughness of the alloy ZL108.

Dynamic fracture toughness of TA15ELI alloy studied by instrumented impact test

The dynamic fracture toughness of TA15ELI alloy with two types of microstructures was studied by instrumented impact test.Charpy specimens with both the 0.2 mm U-notch and the a/W = 0.2 pre-crack were adopted to compare notch sensitivity in the two microstructures.The result shows that the specimen with Widmanst？tten microstructure exhibits a better dynamic fracture toughness and lower notch sensitivity than that with lath-like microstructure.Fracture surfaces in the case of the two microstructures are analyzed to have a ductile and brittle mixed feature under dynamic loading.The fracture surface of lath-like microstructure is composed of dimples and tear ridges,while that of Widmanst？tten microstructure is covered with rough block-like facets and dimples and tear ridges.The α phase boundaries and α/β interfaces act as locations for void nucleation and crack arrest and deviation.The decrease in width of α phase lamellae leads to the increase in the amount of boundaries and interfaces,which causes the increase in the consumption of impact energy and results in the improvement in dynamic fracture toughness.

Effect of microstructure on impact toughness of TC21 alloy

The impact toughness of TC21 alloy after different types of forging and heat treatments was studied. The results show that heat treatment at 915 ℃ for 1 h followed by air-cooling can achieve the highest impact toughness. The crack propagation path of bimodal microstructure is different from that of lamellar microstructure. Boundaries of primary α grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature,the volume fraction of primary α phase decreases and the nucleation sites of microcrack at the primary α phase boundaries also decrease,the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on α/β interface,or the boundary of colony,and/or grain boundary α phase. The crack propagates cross the colony,or along the colony boundary,and/or along β grain boundary. The crack propagation path of lamellar microstructure is dependent on the size,direction of colony. The crack path deflects at grain boundaries,colony boundaries,or arrests and deviates at α/β interface because of crisscross α lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.

Microstructure and mechanical properties of a new high-strength and high-toughness titanium alloy

In order to develop a new titanium alloy with a good combination of strength-ductility-toughness,a nearbeta titanium alloy was designed based on the already widely used Ti-1023 alloy.To avoid beta fleck occurring in the microstructure,the new Ti-Al-Fe-V(Cr,Zr) alloy has been made through decreasing the content of Fe,based on molybdenum equivalency and Bo-Md molecular orbital method(a method for new alloy designing based on the molecular orbital calculating).After primary design computation,Ti-Al-Fe-V(Cr,Zr) alloy was optimized as Ti-3Al-4.5Cr-1Fe-4V-1Zr finally.The microstructure and tensile properties of this alloy subjected to several commonly used heat treatments were investigated.The results show that the tensile strength of the alloy after solution treated below the β-transus temperature comes between 850 and 1100 MPa,with elongation in the range of 12.5 %-17.0 %.In solution-treated and solution-aged samples,a low-temperature aging at 500 ℃ results in the precipitation of finer α phase.With the increase in aging temperature,the secondary α phase becomes coarser and decreases in amount.Thus,it will lead to the increase in tensile ductility,but reduction in strength.Eventually,after modulated aging treatment,the alloy can obtain highstrength level with acceptable ductility.The tensile strength of the alloy can achieve 1273 MPa,with an elongation of 11.0 %.At the same time,the fracture toughness(K_(IC)) of the alloy achieves 83.8 MPa·m^(1/2).It is obvious that the newly designed alloy has achieved a good blend of strength-ductility-toughness.

New high-strength Ti–Al–V–Mo alloy: from high-throughput composition design to mechanical properties

The high-throughput diffusion-multiple technique and thermodynamics databases were used to design new high-strength Ti alloys. The composition–microstructure–property relationships of the Ti64–xMo alloys were obtained. The phase fraction and composition of the α and β phases of the Ti64–xMo alloys were calculated using the Thermo-Calc software. After aging at 600℃, the Ti64–6 Mo alloy precipitated ultrafine α phases. This phenomenon was explained on the basis of the pseudo-spinodal mechanism by calculating the Gibbs energy curves of the α and β phases of the Ti64–xMo alloys at 600℃. Bulk forged Ti64–6 Mo alloy exhibited high strength and moderate plasticity after α/β-phase-field solution treatment plus aging. The tensile properties of the alloy were determined by the size and morphology of the primary and secondary α phases and by the β grain size.

Effect of Post Weld Heat Treatment on Microstructure and Fracture Toughness of Friction Welded Joint

The effect of post weld heat treatment on the microstructure and fracture toughness of friction welded joints of Ti-6.5Al-1Mo-1V-2Zr alloy was studied. The experimental results show that equiaxial grains were formed at the center of the weld metal while highly deformed grains were observed in the thermomechanically affected zone. The fracture toughness of the weld metal was lower than that of the thermomechanically affected zone under as-weld and post weld heat treatment conditions. With increasing temperature of post weld heat treatment, the fracture toughness of weld center and thermomechanically affected zone increased. The fractographic observation revealed that the friction welded joints fractured in a ductile mode.

BASCA热处理对TC10钛合金组织与断裂韧性的影响

本工作对TC10钛合金进行BASCA热处理(β退火+缓慢冷却+时效),通过改变BA温度(β退火温度),研究BASCA热处理对TC10钛合金微观组织与断裂韧性的影响。结果表明:BA温度对合金微观组织与断裂韧性起到决定作用,随着BA温度升高,合金微观组织类型由等轴组织转变为片层组织,断裂韧性不断增加,最大值为77 MPa·m^(1/2)。此外,研究了不同类型组织的裂纹尖端区域塑性变形量、裂纹扩展路径以及断口微观形貌,进一步揭示断裂机制。与片层组织相比,等轴组织的裂纹尖端区域塑性变形较大,裂纹扩展路径曲折程度较小。等轴组织的断口形貌光滑平顺,主要由韧窝构成;片层组织的断口形貌凹凸起伏明显,韧窝数量与尺寸均减小,并出现撕裂棱、空洞以及二次裂纹。

12 mm厚TC4钛合金激光-MIG复合焊接头组织与性能研究

目的探究TC4激光-MIG复合焊接头显微组织与基本力学性能之间的联系,分析接头不同区域的断裂行为。方法利用激光-MIG复合焊制备TC4钛合金对接接头,采用光学显微镜和扫描电镜观察接头焊缝区、热影响区及母材的显微组织,在室温下进行了显微硬度测试、拉伸性能测试与断裂韧性测试,并对试样断口进行了观察分析。结果接头的焊缝区组织为粗大的β相柱状晶,晶内纵横分布着αʹ针状马氏体和针状α相,靠近焊缝一侧的热影响区则由针状αʹ相、α集束与少量细小的块状α相构成。随着远离焊缝中心,母材侧热影响区组织转变为块状的α相、少量α集束及初生β相,并最终趋于与母材组织相似。热影响区的显微硬度值达到最大,这是因为该区域存在比焊缝区更为细小的针状αʹ相。接头的平均抗拉强度和断后伸长率分别为1020.22 MPa和7.38%。接头在拉伸时主要在焊缝区发生断裂。焊缝区展现出比母材区和热影响区更优异的断裂韧性,平均值为87.14 MPa·m1/2,焊缝区内纵横交错的网篮组织与集束是其断裂韧性较高的主要原因。结论在TC4钛合金的激光-MIG复合焊过程中,针状α相和αʹ马氏体的存在会提高焊缝的显微硬度和断裂韧性,但相较于母材塑性没有提升,通过调控焊缝区显微组织结构,可以获得所需性能的接头。

准β锻造和β热处理对TC11组织和断裂韧性的影响

研究了准β锻造和β热处理对TC11钛合金显微组织和断裂韧性的影响。结果表明TC11钛合金经准β锻造及β热处理后断裂韧性相较于魏氏组织明显提升,延伸率提升约50%,断面收缩率提升约20%,KIC提升约20%。由于准β锻造改变了层片状α相的析出形态,形成了取向差异化明显的束集,抵抗裂纹扩展的组织协调性更佳,从而使得断裂韧性更优;而魏氏组织试样主要以晶界和大片次生α相增加裂纹扩展曲折性。

时效温度对TA10钛合金微观组织以及冲击韧性的影响

对TA10钛合金进行固溶时效处理,研究TA10钛合金在不同时效温度条件下微观组织形貌与冲击韧性二者之间的关系。研究发现:在固溶条件下,合金组织结构主要由α′相和初生α相组成,再经时效处理后,组织中初生α相的形貌以及体积分数均未出现明显变化,且有大量次生α相析出,并有明显的β转变组织形成。合金经固溶时效处理后的冲击韧性,较固溶处理后的数值更低,而在固溶时效条件下,随着时效温度的逐步提高,合金的冲击韧性整体呈现出降低的趋势,但整体降低趋势并不十分明显。发现无论是固溶还是固溶时效条件下,其冲击断口形貌均是由大量韧窝形貌构成,且固溶条件下,冲击断口中的韧窝数量更多,其断口整体较为平稳,同时发现断口中空洞的数量和尺寸在时效温度升高的过程中也不断增加。

Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys

The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.

Study on dynamic fracture behavior of TA15ELI alloy under mode-Ⅱ loading by caustics method

The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa.m1/2 and 32.6 m/s, respectively. SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA 15ELI alloy in dynamic mode-II fracture. Shear localization was observed in the vicinity of the crack initiation area.

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.

TC4钛合金EBW和TIG焊接头断裂韧性对比分析

采用电子束焊和氩弧焊方法制备的TC4钛合金接头,在4种温度条件下,测试了其焊缝、热影响区及母材的断裂韧性,并结合断口形貌、硬度分布及微观组织对比分析了断裂韧性测试结果.结果表明,TC4钛合金TIG焊接头焊缝及热影响区的断裂韧性优于钛合金母材,CTOD值随温度下降而降低;相较之下,EBW焊接头焊缝断裂韧性值较母材更低,热影响区断裂韧性与母材较接近,温度变化对其CTOD值无显著影响.钛合金TIG焊接头焊缝区域具有较低硬度值,其网篮状α相和较低比例的马氏体分布是其断裂韧性较高的根本原因;而钛合金EBW焊接头焊缝中针状马氏体的分布导致其局部硬度较高,并降低了断裂韧性.

内壁强化对TA16钛合金传热管断裂韧度的影响

采用TA16传热管制备的直流蒸汽发生器是新一代核动力装置的核心部件。为提高管材的内壁质量,目前采用内绞、喷砂等数道表面加工程序,生产成本大幅升高。为简化工艺流程,降低生产成本,开展针对TA16管材的内壁强化技术研究。同时为了确定内壁强化对管材断裂韧性的影响,提出一种含双边轴向裂纹管(DEAT)试样,通过弹塑性有限元分析建立DEAT试样的应力强度因子K和J积分的计算表达式,通过建立的试验方法完成内壁强化前后TA16传热管的J-R曲线测试,获得内壁强化前后TA16传热管的断裂韧性参数。试验后采用SEM、EBSD、TEM及纳米压痕分析内壁强化对管材微观组织和性能的影响。研究结果表明:强化后TA16管内壁从表面沿径向形成100~200μm的纳米超细晶区和孪晶变形区,显著提高了管材的内壁质量,但由于表层的硬度升高,导致传热管的断裂韧度整体水平有所下降。研究结果可为TA16管材的质量提升、安全性评价及性能退化评估奠定技术基础。

TC21钛合金固溶时效处理对合金强韧化性能的影响

本文研究了TC21钛合金经两相区固溶处理快速冷却后,合金显微组织、室温拉伸性能及室温冲击性能随时效制度的变化规律。研究表明,TC21钛合金在两相区进行固溶处理时快速冷却后,随着时效时间的延长,合金组织中初生α相尺寸增大,但抗拉强度有所下降,塑性变化不明显。随着时效温度的升高,合金组织中的初生α相尺寸增大,合金的强度明显下降,塑性提高。TC21钛合金在两相区固溶处理快速冷却下,通过900℃保温120 min、水冷(WC),600℃保温360 min、空冷(AC)的热处理工艺,可使合金具有最佳的强度和塑性匹配。

Ti-55531钛合金室温强-塑-韧匹配化热处理工艺研究

目的采用Ti-5Al-5Mo-5V-3Cr-1Zr(Ti-55531)钛合金,研究了不同热处理工艺条件下室温强-塑-韧性能的匹配关系,为满足不同强度、断裂延伸率、断裂韧度综合服役性能要求提供热处理工艺参考。方法在单相区固溶+时效、双相区固溶+时效2种制度下进行了热处理试验,分析了不同单相区固溶冷却方式(空冷、炉冷)和时效温度、双相区固溶温度等条件下的室温拉伸性能(抗拉强度σb、断裂延伸率A)和断裂韧度KIC,揭示了Ti-55531钛合金室温强度、塑性、断裂韧度的匹配关系。结果经单相区固溶+空冷+时效处理得到了细片层状次生αs相,随时效温度的升高,αs相尺寸增大,抗拉强度降低,延伸率和断裂韧度升高;经单相区固溶+炉冷+时效处理得到了较粗的α片层,随时效温度从500℃升高至600℃,α片层尺寸增大,抗拉强度降低,延伸率和断裂韧度升高,但呈现出较高的脆性;随着双相区固溶温度的升高,初生αp相尺寸显著降低,促进后续时效处理过程中析出了更细小的次生αs相,提高了强度,降低了延伸率和断裂韧度。结论得到了2种能够实现良好强-塑-韧性能匹配的热处理工艺路线:1)850℃/1 h固溶后炉冷至600℃保温8 h,可得到片层组织以及较高的断裂韧度(KIC=110.01 MPa·m1/2)、良好的强度(σb=1111 MPa)和断裂延伸率(A=9.69%);2)810℃固溶+空冷+600℃/3 h时效,可得到初生αp+次生αs相的双态组织,实现了高强度(σb=1287MPa)和高断裂延伸率(A=12.76%),同时断裂韧度达到60.4MPa·m1/2。

Phase Transformation in a β-Ti Alloy with Good Balance Between High Strength and High Fracture Toughness

This article studies the phase transformation of the metastable β-Ti-A1-Mo-V-Cr-Zr alloy （Ti-1300） to disclose the morphological reason for its high strength and high fracture toughness. It has been found that its ultrahigh strength （ultimate tensile strength exceeds 1 400 MPa） owes mainly to the spheroidization of the α-phase, while the high fracture toughness （exceeds 81 MPa·m^1/2） to the special lath-shaped α-particles. Compared to the needle-shaped second α-articles, the coarser lath-shaped ones remove the stress concentration at the lath tips and consequently benefit improvement of fracture toughness. The article also describes shape evolution of the α-particles during aging thermodynamically and kinetically, and suggests an optimized aging processing to achieve an ideal balance between high strength and high toughness for this alloy.